The widespread use of exercise as a way to improve overall health, achieve better recoveries and better functional activity of mind and body underlines the importance of non-invasive quantitation of oxygen delivery and utilization in skeletal muscle. We have previously developed and evaluated portable, economical spectrophotometers for giving qualitative measures of muscle performance and obtained signif aboutcant comparative evaluations of normal and abnormal populations. Here we propose novel theoretical developments that permit quantitation of muscle function with overlying layers of fat and melanin by using a finite element representation of photon diffusion. We will also construct an unique, portable frequency domain imager (IQI), test it in models and compare it with previous non-quantitative imagers. This new instrument quantifies the following skeletal muscle properties: 1) Oxygen saturation, 2) tissue oxygen gradients, 3) metabolic rates (recovery rates), and 4) microscopic and macroscopic heterogeneities. Co-registration of muscle functional images of localized hypoxia by NIR IQI imaging of HbO2+ M bO2 - Hb + Mb, rH MRS of Mb O2 about Mb and 3iP NMR of PCr about Cr will afford an evaluation of NIR imaging performance. These technologies are used to study skeletal muscle performances in "well known" populations of elite (Olympic) athletes, "normals" and geriatrics in order to characterize and quantify the four parameters of these populations. Three populations of abnormal characteristics involve vascular and bioenergetic disability, forms a focus of clinical studies based upon our early success in a cytochrome bct deficiency (Eleff, et al (1984) see Ref 8)). A novel criterion for early detection of mitochondrial desease (MD) is based upon imaging an anomalous response of MD's to exercise, a "paradoxical" oxygenation (PO) instead of the usual deoxygenation at maximal exercise. The goal in all studies is to determine the effectiveness of the quantitative imaging of exercise or disease induced hypoxia. Basic knowledge of tissue oxygen gradients and their bioenergetic consequences in healthy and diseased skeletal muscle is essential to understand, evaluate and quantify therapeutics for normal and diseased muscle. Our especial goals are improving geriatric performance and detecting mitochondrial disease. See Appendix A for Glossary.